The present invention relates to thickener compositions useful for thickening aqueous systems such as latex paints, and, more particularly, to rheological agents and thickeners for aqueous systems in the form of a mixture of clay, polymeric particles and starch-type materials which provides superior thickening efficiency, roller spatter resistance characteristics, and improved flow properties at substantially lower costs than with currently available thickeners.
Rheological additives are substances which, when added to a system, change the system's rheological properties such as flow, viscosity, leveling, sag resistance, settling and the like. These additives include rheology modifiers, viscosity modifiers, viscosifiers, thickeners, gellants, thixotropes, anti-sag agents, anti-setting agents, suspending agents, emulsifiers, thinners and surfactants.
Commercial thickeners for latex paints available today are mostly cellulosics which thicken the water carrier of the paints. Such thickeners are relatively expensive and, at the same time, do not provide the necessary rheology to achieve better roller spatter resistance and flow properties.
Aqueous compositions which generally employ thickeners include paints and other coating compositions such as coatings for paper, inks, and the like. Most of these aqueous compositions require the use of a thickening material to improve rheological properties such as to obtain a proper viscosity in order to maintain pigments, binders and other solids dispersed, suspended or emulsified therein, to obtain suitable flow properties, and to obtain good compatibility.
Among the well known rheological additives are clays, organoclays, fumed silica, organic and polymeric thickeners. These rheological additives are described in Van der Watt and Godman in Clay and Clay Materials, Volume 9, pate 568 (1960) and McCormick, Gok and Schulz, in Encyclopedia or Polymer Science and Engineering, Volume 17, page 730 (1989).
Clay minerals are a group of minerals essentially composed of hydrated aluminum silicates. Among these are montmorillonite, kaolin, illite, saponite, bentonite, hectorite, beidellite, stevensite, attapulgite and sepiolite. These minerals are described in Applied Clay Mineralogy, R. E. Grim, McGraw Hill, NY (1962). One important type of clay mineral is hectorite, which is obtained from various deposits. Hectorite deposits typically contain about 50% hectorite by weight, the other components being mainly calcite and dolomite and sometimes feldspar. The hectorite deposit is usually beneficiated by removing the impurities, for example, by preparing a water slurry, centrifugation, separation of the purified clay and drying the clay by various drying methods such as drum drying, oven drying or spray drying.
Among the polymeric rheological additives are guar gum, tragacanth gum, pectin, xanthan and alginate, which are natural thickeners, hydroxyethylcellulose and methyl cellulose, which are modified cellulosics and synthetic polymers or copolymers of ethylenically unsaturated carboxylic acids and their ester derivatives, such as poly (acrylic acid), poly acrylamide and maleic anhydridestyrene copolymers. Such thickeners are described, for example, in U.S. Pat. No. 4,507,426. U.S. Pat. No. 3,769,257 describes the use of cellulosic ethers in latex based paints. U.S. Pat. No. 4,558,079 describes the use of hydrophobically modified cellulosic materials as a thickener for joint compounds which are used in the construction of walls and ceilings. U.S. Pat. No. 4,226,754 describes a synthetic copolymer which is the reaction product of methacrylic acid, methyl methacrylate and vinyl acetate and its use as a thickener for water based coatings. Another type of synthetic thickeners is based on hydrophobically capping a water soluble polymer. For example, U.S. Pat. No. 3,770,684 describes a thickener based on a water soluble polyether capped with insoluble hydrocarbon residues and their use in latex paint compositions. U.S. Pat. No. 4,079,028 describes polyurethane-based copolymers which have hydrophobic end groups connected by hydrophilic polyether groups.
Polymeric thickeners and clay minerals have been used in formulations for various products. For example, U.S. Pat. No. 4,657,943 describes a thickening agent useful in textile padding and oil drilling muds containing an acrylate polymer and an aluminum silicate, montmorillonite clay or bentonite which shows a synergistic effect in increasing the viscosity of the system under alkaline conditions. Thickeners are used in a host of products such as paints, varnishes, enamels, waxes, oil drilling fluids, inks, adhesives, sealants, cosmetics, paper coatings, textile backings, bath emulsion systems, cleaners and polishers and detergents.
An important use of thickeners is in latex paint compositions which are used extensively for coatings and decorations. It is desired that such coating systems will be flowing during their application, but that they do not sag after they have been applied. It is further desired that, while being applied, these coatings remain on the substrate and not spatter around the application area. It is still further desired that such coating systems will be stable at elevated temperatures and for a sufficient period of time, so that their properties do not change upon storage. The thickener may be added to the coating system either during the grinding stage or after the pigment has been dispersed, i.e. as a post-additive. Polysaccarides such as hydroxyethyl cellulose, have dominated the latex paint thickener market almost since the inception of latex paint. Hydroxyethyl cellulose is a nonionic water soluble material that thickens and stabilizes many water-based paint systems. Hydroxyethyl cellulose, however, suffers from some disadvantages. For example, hydroxyethyl cellulose is costly, is subject to enzyme degradation and produces poor spatter resistance. Hydroxyethyl cellulose thickeners are subject to bacterial and enzyme attack which results in loss of viscosity and other flow properties, and therefore require use of anti-biodegradation preservatives in paint. Furthermore, the thickeners swell rapidly in water to form lumps which are not readily dispersed. The proper addition of hydroxyethyl cellulose thickeners requires, therefore, careful and slow addition, resulting in long mixing and dilution times, thereby adding steps to the coatings preparation process, which in turn increase the coatings processing cost.
The search has therefore continued for a thickener which (1) can be employed as a post-additive, directly to aqueous compositions, particularly latex paint compositions, (2) is less biodegradable, and (3) yields a lower cost/performance ratio with respect to the overall manufacturing and rheological properties of a latex paint composition into which it is incorporated, such as in addition to thickening action, spattering resistance, leveling and when compared to cellulosic materials.
One solution to this problem has been the development of a synthetic polymers in the form of a reaction product of methacrylic acid, methyl methacrylate and vinyl acetate in the proportions specified in U.S. Pat. No. 4,226,754, the contents of which are specifically incorporated herein by reference. We have found, however, that thickener blends having higher concentrations of such synthetic polymers are both costly to use and often produce poor paint stability.
The synthetic polymers also require high concentrations to provide the required rheological properties. In many applications, the aqueous system containing the thickener is exposed to temperature in the range of 20.degree. C. to 100.degree. C. The increase in temperature causes generally a decrease in viscosity and stability of the aqueous composition.
Smectite-type clays such as montmorillonite, bentonite, beidellite, hectorite, saponite and stevensite and a process for producing organopilic clay gellants using these smectite-type clays are described more specifically in U.S. Pat. No. 4,742,098. Such clays, particularly hectorite, have been found useful in paints as thickeners as well as in polymeric thickeners, but to date these products have exhibited poor roller-splatter, poor efficiency characteristics or poor stability.
Attempts to improve the known rheological agents and thickeners by using blends of hectorite clay and polymeric particles have thus been unsuccessful in increasing stability, product efficiency, roller spatter resistance, easy dispersal and cost. For example, too much hectorite has been found to result in poor roller spatter resistance and low product efficiency, while too much polymeric material has been found costly and results in poor paint stability.
Thus, there has been and still is a need for a thickening composition which imparts increased viscosity, sag resistance, spatter resistance and heat stability to water based coatings systems. There is also a need for a low cost thickener which will be easy to utilize in various types of formulations, especially as a post additive, added directly to aqueous systems to impart these properties.
We have recognized that a third component in the claypolymer mixture will lower the ratio of polymeric particles to clay and thereby achieve a cost effective, yet stable rheological agent which is also superior to the two component product. We further recognized that such an additional component must be a starch or a starch derivative, including chemical modifications to the starch through esterification, ethoxylation and carboxylation.
Starch is the principal component of most seeds, tubers and roots and is produced commercially from corn, wheat, rice, tapioca, potato, sago and other sources. Starch is a mixture of linear (amylose) and branched (amylopectin) polysaccharides. It is used extensively in the paper, textile and adhesive industries. Starch may be modified by oxidation of the hydroxyl groups to give aldehydes, ketones or carboxylic acids. Other derivatives are produced by etherification, esterification and graft polymerization. Hydroxy propyl and other hydroxyalkyl starches are used as additives in salad dressing and other food thickening applications.
It is an object of the present invention, therefore, to provide a composition containing an aqueous starch thickener for latex paints which provides increased viscosity and improves roller spatter resistance while maintaining good paint stability.
It is still another object of the present invention to provide an aqueous starch thickener composition which is less expensive than cellulosic thickeners and yet achieves increased product efficiency, while being easily dispersed in the grind and/or as water gel solutions.
The foregoing objects have been achieved in accordance with the present invention by the use of a clay/polymeric particle/starch mixture as rheological agents and thickeners for aqueous systems such as latex paints and other latex systems.
The present invention is directed to a composition of matter in which the clay is smectite, including hectorite or spray-dried hectorite, which composition interacts with latex, pigments and the like to yield higher viscosity.
More specifically, the present invention relates to a thickener composition for water based systems comprising:
(i) a clay which includes smectite, montmorillonite, bentonite, hectorite, kaolin, illite, saponite, attapulgite, sepiolite and the like and mixtures thereof; PA1 (ii) a starch which includes corn starch, wheat starch, rice starch, tapioca starch, potato starch, sago starch, yucca starch, barley starch and the like, derivatives thereof including modifications through esterification, ethoxylation, carboxylation, grafting and mixtures thereof; PA1 (iii) a polymer or copolymer which is the polymerization product of ethylenically unsaturated carboxylic acid monomer, modified ethylenically unsaturated carboxylic acid monomer and/or vinyl or modified vinyl monomer such as vinyl acetate.
The composition of the present invention further utilizes polymeric particles and commercially-available LORAMA 3030 starch or another suitable starch such as corn, yucca, barley and potato starches and derivatives thereof, including modifications through esterification, ethoxylation and carboxylation.
The invention further relates to an aqueous coating system comprising latex, pigments, extenders and fillers and from about 0.05% to 15% thickener composition comprising a clay/starch/polymer composition.
The invention is also directed to a method for preparing the thickener composition and the coating system, comprising mixing the clay, the starch and the polymer to obtain a blend, and adding the blend either during the grinding step of the coating preparation or as a postadditive composition to the coating system.